1. Field of the Invention
The present invention concerns transmission systems for high-speed track-laying vehicles or vehicles with non-steerable wheels.
2. Description of the Prior Art
High-speed track-laying vehicles, generally intended for military uses, must as far as possible be capable of moving as fast as possible on roads, of crossing the most severe obstacles and of retaining under all circumstances the greatest possible mobility or "agility".
These vehicles must be capable of achieving speeds well in excess of 60 kph and even as high as 80 kph, which distinguishes them from other track-laying vehicles such as those used in public works, which as a general rule do not have to move as fast.
In both cases steering the vehicle is based on the general principle whereby the relative speed of the tracks is varied, but it will be readily understood that the transmission systems used are not designed in the same manner, given the much more severe conditions that prevail in the case of steering high-speed track-laying vehicles because of the high power that passes to the tracks. In the case of earthmoving plant, this power is always relatively modest.
What is more, accurate control at high speed requires a steering system procuring continuous variation of the turning radius.
For these reasons there is an increasing tendency to use for steering modern vehicles, and in particular tanks, hydrostatic transmission systems, in particular transmissions incorporating a variable delivery pump (with axial pistons and inclinable plate, or with inclinable or non-inclinable barrel) and a fixed capacity hydraulic motor. Fitting these is facilitated by the fact that the same oil can be used for the steering transmission and the gearbox, and this oil can be the same as that used for the diesel engine.
Thus there are known transmission systems for high-speed track-laying vehicles or vehicles with non-steerable wheels comprising a gearbox equipped with a hydrodynamic torque converter functioning in a power splitting arrangement, steering means featuring continuous variation of the turning radius and reversing gearing associated with epicyclic output gears.
Various techniques have already been put forward for improving the performance of known transmissions and reference may usefully be had to French patent applications Nos 2 450 189 and 2 602 481 from the same inventor as the present application and the contents of which are hereby incorporated by way of reference.
In French patent application No 2 450 189 it is proposed to use on the steering shaft a variable speed drive the primary of which is driven either from the primary of the torque converter of the transmission or from the secondary of this converter, according to which of these two members is rotating faster, so conferring on the vehicle the minimal turning radius.
In French patent application No 2 602 481 it is further proposed to provide a transmission with a single-output steering device: in this case the output of the gearbox is connected to the annulus gear of a first epicyclic gear (with fixed sun gear and planet carrier connected to the rotating part of the braking device) and the steering device comprises a rotary member coupled to the sun gear of a second epicyclic gear with the same ratio (with the planet carrier fastened to that of the first gear and an annulus gear connected to the motion output shaft which extends to the braking and steering devices).
The state of the art is further illustrated by French patents Nos. 2 606 713, 2 540 058, 2 468 790, 2 395 873 and 2 377 923, filed by this applicant, and British patents Nos. 1 456 717 and 2 006 899.
In parallel with this work there is considerable research on drive engines, especially on supercharged diesel engines.
In particular, researchers are attempting to develop turbines with separate combustion chambers driving a compressor ("hyperbar" system): as yet the cost of these solutions would seem to be prohibitive, however. A more promising line of research is into using two-stroke diesel engines: in particular it seems possible, by appropriately adapting the exhaust valves, to achieve a high degree of supercharging as a result of a higher pressure of the exhaust gas driving the supercharger turbine, even at low engine speeds, which results in constant power from speeds in the order of half the maximum speed.
An object of the invention is to provide a transmission that is particularly well suited to engines supplying constant power over a wide speed range, for example in a ratio of 1:2.
Another object of the invention is to provide a transmission of simple and rugged design enabling the number of gear ratios to be reduced while improving the performance of the vehicle using it.
A further object of the invention is to provide a transmission enabling three modes of operation to be established, apart from the neutral and braked neutral positions, the first of these modes being a power spitting mode, the second a direct drive mode and the third an overdrive mode.